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UNIVERSITY BULLETIN 

LOUISIANA STATE UNIVERSITY 

AND 

AGRICULTURAL AND MECHANICAL COLLEGE 

VOL. Xlil- N. S. AUGUST, 1921 No. 7 



PLANT DISEASE INVESTIGATIONS 

AT THE 

AGRICULTURAL EXPERIMENT 
STATION 

BY 

C W. EDGERTON, 

PLANT PATHOLOGlSt. EXPERIMENT STATIONS 




BATON ROUGE.^ LOUISIANA 
PUBLISHED BY THE UNIVERSITY TEN TIMES A YEAR 



ENTERED AS SECOND CLASS MATTER, AT BATON ROUGE, UNDER'THE 
ACT OF JULY 16, 1S94 



Wonograp*! 



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Plant Disease Investigations at the Agricultural 
Experiment Station. 

C. W. Edgerton 

Very few men, even those interested in agricultural pursuits, 
realize the actual loss caused by plant diseases to the farmers 
of this country. Short crops follow one another so frequently 
that the average farmer considers them as normal. If a crop 
fails, or is shorter than usual, he more than likely lays it to the 
weather or some other unavoidable circumstance. He very 
seldom realizes or appreciates that every crop has from one to 
several diseases which very frequently are instrumental in re- 
ducing the yields and consequently the profits. 

In many instances, short crops and crop failures are due to 
the attack of plant diseases. Very o_ftei3^iaie presence or absence 
of a disease is theJ^eis\i»^'f^c1m'hetwBen ^failure or success with 
a crop. In many insta-tices, .tli^.presence ^f a disease has been 
the primary cause for .discontinuing certliin crops or certain 
varieties. There. are a numbei-^o^/^i^MP^i^tances in Louisiana. 
The pear was once. widely g^WiTalicf'-ilad'the promise of being 
an especially vahtatile" fruit in this state, but the blight found 
its way into the state and completely eliminated the pear from 
a commercial standpoint. The Bliss Triumph potato, the potato 
best suited for Louisiana conditions, has been practically replaced 
in the Baj^ou Lafourche district by other varieties which are 
in many ways inferior, for no other reason than that the mosaic 
disease was reducing the yields to such an extent that potatoes 
were not profitable. The citrus canker disease, which gained 
entrance to this state in about 1911, has practically been respon- 
sible for the abandonment of the grapefruit and sweet orange 
industry. 

Estimates of the actual crop losses due to diseases are at 
present being made by the United States Department of Agri- 
culture and State Experiment Station officials. In the last 
report of the Plant Disease Survey covering the year 1919, it is 
estimated that the financial loss to the whole countrv was in the 



billions of dollars. In Louisiana, the percentage of loss is greater 
than in many other states on account of the heavy rainfall and 
mild winters, factors which favor the development of disease. 
Estimates of the losses in Louisiana during 1919 include the 
following: oats, 3%, or an actual loss of 51,000 bushels; corn, 
11%, or a loss of 4,001,000 bushels; Irish potatoes, 11.5%, or a 
loss of 208,000 bushels; tomatoes, 62%c ; sweet potatoes, 29%, or 
a loss of 2,573,000 bushels: cotton, 10% , or a loss of 33,000 bales. 
The loss to the sugar crop has not been estimated as carefully, 
but with the new mosaic disease, the actual loss is well above 10%. 
It is not possible to estimate losses accurately and there is good 
reason to lielieve that some of the above estimates are too low. 
There is little doubt that tlie corn estimate is too low. The 
loss caused by the corn root rot disease alone averages more 
than 10 per cent each season. 

The season of 1919 was not different from otlier seasons in 
regard to the prevalence of disease. Some very serious losses 
to the 1921 crop have already been observed. The 1921 oat crop 
of Louisiana was reduced about twenty per cent by a severe 
infection of leaf or crown rust, due to tlie fact that the farmers 
had to bring in a large amount of seed of non-resistant varieties 
from Texas. Also, the Irish potato crop in some of the in-incipal 
potato sections was reduced from twenty to fifty per cent by the 
mosaic disease. 

The plant disease problem becomes more important as time 
goes on. In a new country with the farms more or less scattered, 
there is little chance for either the introduction or spread of the 
important infectious diseases. It is not uncommon to hear tlie 
older inhabitants of a country say tliat when they were young, 
crops were not affected by these various trouhles. In many in- 
stances, they are more or less nght because at that earlier period 
the fields were more isolated. Many of the diseases had not been 
introduced and there was but little chance for the spread of those 
that were present. 

With an increased acreage of any crop, the plant disease 
problem becomes more important for the same reason that the 
human disease problem is more important in the crowded tene- 
ment districts of the cities than it is in the country. As th-; 



acreage of a crop increases, the fields become laro^er and closer 
together and there is a better opportnnity for the diseases to 
spread from field to field. Purtliermore, as agricnltnre becomes 
more intensified, the soil tends to become severely infested with 
the varions organisms which produce the diseases. Also, new 
diseases are more readily introduced from other countries or 
other localities. During the past ten years, a number of new 
diseases have found their way into Louisiana. Some of the more 
important ones are the citrus canker, the sugar cane mosaic, the 
cabbage yellows and the sweet potato stem rot. It is reasonable 
to expect that more diseases will be introduced and there is little 
doubt that all the diseases will become more important unless the 
proper control measures are found and put into practice. 

Before beginning the discussion of the disease problem and 
the work that is being done to solve it, it is well to make clear 
what is meant and included by the term plant disease. Strictly 
speaking, a condition of disease exists when any organ of an 
individual is not functioning normally or when the various 
organs are not working in harmony. From a practical stand- 
point, however, a condition of disease exists when a crop is not 
producing the maximum that could be expected from the soil 
under the existing weather conditions. To the farmer or planter, 
a disease is important in the same proportion as it reduces the 
crop. A disease that affects only a few plants in a field may be 
extremely interesting to the technical pathologist, but it is only 
of minor interest in the field of economic patholog}^ On this 
account, human pathology and plant pathology- do not have 
exactly the same end in view. An uncommon disease of man 
must be considered and be fully studied and understood by the 
medical man because the saving of the individual is the important 
thing, but in plant pathology the individual is not usually so 
important. The whole crop, many thousands of individuals, is 
realh^ the unit to be considered. As a matter of fact, economic 
pathology has as its main object the control or prevention of 
epidemics rather than the control of disease on the individual. 



6 

CAUSE OF PLANT DISEASES 

In considering plant diseases, it is necessary to keep in mind 
the distinction between disease and the cause. The disease is the 
disturbance of some function of the plant and may be produced 
by a number of different things. Tlie major portion of the plant 
diseases, which are common and of particular importance, is 
caused by the attack of forms of vegetable and animal life, or 
is the result of growing in an unfavorable environment. This 
possibly does not include all troubles, but if the word environment 
is used in its widest sense, including all conditions existing in the 
soil and air, most plant troubles of which the causes are known 
can be placed in these classes. 

The vegetable forms which are responsible for plant diseases 
include various bacteria, fungi and a few of the higher plants. 




Fig. 1. Rot that has developed over night in a box of Louisiana 
strawberries, the result of packing defective berries with good ones. 
The Louisiana growers lose thousands of dollars each year in this way. 




Fig. 2. Insect pests attacked and killed by fungi; on the left, white 
fly on orange leaf; on the right, plant lice on mustard. Notice that 
practicallj' all of the insects are killed and are covered with the mold 
of the fungus. 

The low plant org-anisms are responsible for a greater part of the 
plant diseases which are spoken of as infectious or contagious 
and these are really the diseases which are of primary impor- 
tance. All the diseases commonly spoken of as rusts, smuts, 
blights, wilts, root rots, etc., the diseases which occur in epidemic 
form and are responsible for a greater portion of the financial 
loss to the agricultural interests, are caused by various species 
of bacteria and fungi. 

These fungous and bacterial forms are also responsible for 
all the storage and transportation troubles which develop with 
perishable fruits and vegetables. The deterioration of these 
products during storage and while being transported to market 
is a problem of extreme importance to a state like Louisiana, 
in which there is a considerable trucking industry. For example, 
a box of strawberries containing defective berries^ will often be 
entirely ruined in twelve to twenty-four hours. Such a box is 



shown in Fig. 1. Hundreds of crates of Louisiana berries go 
like this every season and finally find their way to the dump pile. 
All fungi and bacteria, however, are not detrimental to the 
farmer. Some forms are particularly benefieial. For example, 
various troublesome insects are often seriously attacked by 
various fungi and are killed in great numbers. In Fig. 2 are 
shown plant lice on mustard and white fly on orange leaves 
parasitized by fungi. The white fly is usually kept in check 
to such an extent by this fungus that it causes but little damage. 

THE WORK OF THE EXPERIMENT STATION 

The investigations on the plant diseases of the state have 
comprised a portion of the work of the Louisiana Agricultural 
Experiment Station for a number of years. Perhaps in no state 
is the need of work along this line of more imi)ortance than it 
is here, on account of our extremely favorable weather con- 
ditions. Every crop is affected by one or moi'c diseases, some 
of considerable and some of minor importanc(\, yet all cutting 
down the yields and reducing the profits of the farmers. It is 
the aim of the department of plant pathology to watch all of 
these troubles as much as is possible and to carry on investiga- 
tions on those which will throw light on the general problems 
of disease development, dissemination and control, or upon those 
which seem to be of particular economic importance. In the 
past few years, investigations have been carried on with a number 
of the important diseases of cotton, sugar cane and certain fruit 
and truck crops. At the present time, the work in progress is 
largely with sugar cane, corn and tomatoes, though some of the 
other crops are being considered to some extent. 

Besides the investigational work, the department is con- 
stantly being called on for information about various diseases 
which are present in the state. This information is always given 
as far as possible. Of course, many diseases are as yet so little 
understood that it is impossible to give any definite information 
regarding their control. 

NATURE OF THE INVESTIGATIONS 

While one of the ultimate objects in the study of any disease 
is to determine the most satisfactorj^ and economical method of 
control, control measures are not usually considered seriously 




Fig. 3. The anthracnose disease of bur clover. The plant on the 
left was inoculated with a pure culture of the causative fungus and has 
been practically killed. 

in the investigational work until the work is well advanced. In 
research work on any disease, the first thing is to obtain all the 
information possible regarding the disease and its cause. If the 
disease is caused by a fungus or bacterium, this organism must 
be isolated if possible, and grown in pure culture in the labora- 
tory. It must be watched on different culture media and at 
different temperatures, and the various factors which tend to 
increase or decrease development must be studied. Then healthy 
host plants must be inoculated artificially (Fig. 3), and a 
thorough knowledge obtained of the methods of infection, the 
period of incubation and the rapidity of the development of the 
disease. Furthermore, the disease must be studied in the field. 
It is necessary to determine how the organism is normally car- 
ried from plant to plant, whether by insects, rain, winds or by 
other means, and also whether the organism gains entrance to 
the host plant through injuries or is able to infect the uninjured 
surface. 

It is also necessary to determine how the organism passes the 
season of the year during which the host is' not present, or at 



10 



least dormant, the winter with some plants and the hot summer 
with others. Some remain alive on the seed, some in a dormant 
condition within the seed, some live over in the soil or upon 
decaying parts of old plants, some pass to other hosts and some 
pass this period in other ways. The different spore forms of 
the parasite must be studied, as each usually has a definite part 
to pla}^ in the dissemination of the organism. Some spore forms 
live only lor a day and must infect the host immediately after 
they mature, while others are capable of living for months or 
years. 

Furthermore, different varieties of the host plant must be 
inoculated with the organism and with dift'erent strains of tlie 
organism. It is often found that certain varieties or even certain 
individuals do not take the disease as readily as others. 'Die 
success that has been obtained with .some of our crop varieties 
is due to the fact that they ai*e more or less resistant to certain 
diseases. For example, the Klondyke strawberry shows consid- 
erable resistance to the common leaf spot disease (Fig. 4), and 
this resistance largely explains the development of tlie straw- 




Fig. 4; The leaf spot disease of strawberry. Any strawberry variety 
which is to be grown in Louisiana must show resistance to this disease. 



11 



berry industry in Tangipahoa Parish. Sometimes, also, it is 
found that different strains of the causative organism do not 
attack different varieties in the same wa}^ All these points, 
and many others, must be considered in studying a plant dis- 
ease. 

After a disease is well understood, then is it possible to 
formulate ideas in regard to control. Knowing the complete 
development of a disease, it is often possible to pick out points 
for attack. Very frequently, the parasite has weak points in 
its development and control measures usualty take advantage of 
these. For instance, certain parasites live over the winter on 
the seed of the host plants and it is often possible to eliminate 
these entirely by treating the seed with disinfectants. The 
smut of oats and the scab of Irish potatoes are easily controlled 
by treating the seed with a formaldehyde- solution. 

In our investigations on the anthracnose diseases of beans 
and cotton, data were obtained which permitted the formulation 
of control measures. The fungus causing the bean anthracnose, 
or pod spot disease, develops rapidly during the spring season 
in Louisiana, but it is not able to endure the summer heat. 
Taking advantage of this, it was found that it is possible to 
grow a seed crop in the fall free of the disease. In order to 
have beans ripen in the fall, it is necessary to plant the seed 
during August. The summer temperature kills out the disease 
entirely before the cooler weather of the fall season arrives. 
Disease-free seed is thus obtained in the fall and can be used 
for planting the main crop the following spring. The cotton an- 
thracnose is very similar in many ways to the bean anthracnose, 
but it cannot be controlled in the same manner. The fungus 
causing this disease develops abundantly during our hottest 
weather. However, the weak point in its development was 
found. The fungus lives over from fall until spring within the 
seed, but it was found that the fungus would not live as long 
as the seed itself. The fungus would readily live until spring, 
but it wovild not live until the second spring. As cotton seed 
remains good for several years, it is seen that the disease can be 
controlled by planting seed that is two years old. 

Thus it is seen that knowledge of a disease often permits the 



12 

formulation of control measures, and the more complete the 
knowledge is, the better is the chance of obtaining them. Facts 
about diseases are often obtained wliicli at the time seem of no 
particular value, but later become important. AVhen considered 
with others which are known or which may be discovered later, 
these facts may form links of particular importance. Further- 
more, facts obtained about one disease may be of considerable 
value in the study or understanding of another trouble or they 
may have particular bearing on the whole disease problem. 
Another point of interest is the relation which often exists l)e- 
tw^en different diseases, spoken of in human pathology as com- 
plications. Very often control measures for one disease must 
include control measures for other diseases or for insect pests. 

PRESENT AND FUTURE WORK AT THE 
EXPERIMENT STATION 

At present, the diseases wliieh are receiving the most consid- 
eration at the Experiment Station include the tomato wilt and 
the diseases of sugar cane and corn. Besides these, a i)lant dis- 
ease survey is made of the state each year and some investigations 
are carried on with the troubles tliat are of immediate impor- 
tance. An example of the latter was the severe outbreak. of 
potato mosaic in the state in the spring of 1921. It was neces- 
sary to furnish the growers with reliable information regarding 
this trouble and with the best control measures as (piickly iis 
possible. 

THE TOMATO WILT 

Investigations on the tomato wilt have been carried on for a 
period of twelve years. The tomato disease problem is of con- 
siderable importance as the tomato is widely grown and is also 
more or less linked up with the present educational system of 
the state. No other vegetable is as important in the garden and 
canning club work as is the tomato. 

There are a number of serious tomato diseases, but the one 
which is apparently doing the most injury is the wilt. This 
disease is caused by a fungus which grows, or at least remains 
alive, in the soil for some time. The fungus attacks the young 



13 

roots and grows up into the stems of the plant, causing the 
internal tissues to blacken. Affected plants of susceptible varie- 
ties do not grow satisfactorily and usually die before a satis- 
factory crop is made. The death of the plants is preceded by a 
gradual yellowing of the leaves. This disease is particularly 
severe in the sandy, bluff and prairie sections of the state. In 
contaminated soils, it is not uncommon for all of the plants in 
a field to die, thus causing practically a crop failure. For 
a while in some sections of the state, it looked as if the canning 
club work with tomatoes would have to be discontinued on ac- 
count of this disease. 

Among the interesting things that have been found is that 
it is possible to select or breed strains of tomatoes which show 
a certain amount of tolerance or resistance to the wilt. Varieties 
differ considerably in regard to their susceptibility. In some 
varieties of tomatoes, the disease develops very rapidly, while in 
others the progress is much slower. When the work on the 
tomato wilt was started, the desirable varieties which were being 
grown in the state were mostly extremely susceptible to the dis- 
ease. By trial, a plant was finally obtained from one of the 
varieties which showed considerable resistance. Seed was saved 
from this plant and planted the following spring in ground 
badly affected with the disease. During that season, a number 
of the most resistant plants were selected for seed. This selec- 
tion work was continued for two or three years until a highly 
resistant strain was obtained. Unfortunately— and this very 
often occurs in straight selection work of this kind — this strain 
was not desirable in any other way than in its ability to resist 
the wilt. Its yield was low and the quality of the fruit was only 
fair. This strain was then crossed artificially with an early 
prolific variety with the hope of obtaining a strain that vv^as 
desirable and also resistant to the wilt. After crossing two 
varieties of any crop, it is necessary to carry the cross through 
several generations before the characters again become fixed. 
Consequently, after crossing the tomatoes, it was necessary to 
grow a large number of plants each season for several years, 
selecting each year for seed purposes only those individuals 
that possessed the qualities which were wanted. Without going 
•into the details or the technique of the work, two strains were 



14 



finally obtained which were high yielding, fairly early and com- 
pared to most other varieties, highly resistant to the wilt dis- 
ease (Fig. 5). 

These Louisiana strains have been tried out very thoroughly 
at the Experiment Station and also have been sent out to various 
parts of the state for trial. ^lany people over the state who 
have had the opportunity to try these varieties are now writing 
to the Station each spring for more seed. Since this work has 
been in progress in Louisiana, resistant varieties and strains 
have been put out at other places, but none of these otluM- varie- 




Fig. 5. The result of breeding tomatoes resistant to the wilt disease. 
The plants on the right are the l^ouisiana wilt-resistant varieties, while 
those on the left are ordinary commercial varieties. 

ties have been as satisfactory for our conditions as the varieties 
wiiich have been produced here. 

During the season of 1921, special plots of these varieties were 
:grown on the Experiment Station and a considerable amount 
of seed saved. This seed will be distributed among the agents 
and garden clubs previous to the 1922 season. Using these varie- 
ties and following the suggestions of the Station in regard to 
rotation and seed bed sanitation, tomatoes can be grown in the 
worst wilt-infested sections. The work has now reached such a 
point that the seed growing should be taken up by private in- 



15 

dividuals or seed companies. It is impossible for the Station to 
supply seed in any large amount to those needing it. 

SUGAR CANE DISEASES 

The sugar cane disease problem is one of extreme complexity. 
The problem can be divided into three main divisions, the mosaic 
disease, the root rot troubles and the deterioration of seed cane, 
yet from a practical and time-saving standpoint, it is almost 
necessary to consider them altogether as one big project. Further- 
more, the problem is closely associated with other agricultural 
sciences, including entomology, soil science and meteorology. For 
a complete solution of the problem, it must be considered from 
all of these various angles. 

The mosaic disease is a disease that has been introduced into 
this state within the past few years, but within the period that 
it has been here, it has spread over a considerable portion of 
the sugar belt. The sugar cane mosaic is one of a number of 
similar diseases found on a wide range of plants. While mosaic 
diseases have been known for a considerable period, no definite 
cause has ever been found for any of them. Considerable knowl- 
edge, however, has accumulated in regard to the development, 
dissemination and methods of control. 

The sugar cane mosaic shows on the leaves in the form of 
light colored stripes. In some countries, the loss caused by this 
disease has been estimated as high as forty per cent, but with 
our Louisiana varieties, the loss is considerably less. The in- 
vestigational work with this disease is being carried on largely 
at the Audubon Park Sugar Station in New Orleans. The work 
includes the study of the effect of the disease on the host, the 
testing out of different varieties for resistance and the selection 
for disease resistance. Breeding for disease resistance with a 
plant that does not produce true seeds is recognized as a doubtful 
proposition at the best and results cannot be expected under 
several years. We have, however, tested out the relative re- 
sistance of our commercial varieties and have developed a method 
of selecting disease-free seed of the L 511 variety for planting 
purposes. Affected stalks of the L 511 cane have red stripes 
upon them, as is shown in Fig. 6, and it is possible to discard 



16 




Fig. 6. The sugar cane mosaic on stalks of L 511 sugar cane. The 
stalks with these stripes can be eliminated at planting time. 

these at planting time. Last season, several of the planters care- 
fully selected their L 511 cane following the suggestions given 
in a bulletin published by the Station, and their cane of this 
variety this season is remarkably free of the disease. 

The deterioration of seed cane is a problem of extreme impor- 
tance to Louisiana, as no other troul)le cuts clown the profits to 
the same extent. This deterioration of the seed forces the plant- 
ers to use several times as much seed as is required in most of 
the tropical countries. A part of this deterioration has been 
found to be due to the same fungus which produces the red rot 
disease in living canes, but this is only one of the many factors 
involved in this problem. Concerning most of the factors, we 
have as yet but little data. 



17 



DISEASES OF CORN 

The diseases of corn which are of particular importance in 
Louisiana are those classed under the general terms of root and 
stem rots. As there are several organisms which seem to be more 
or less responsible, this is also a complex problem. There is a 
bacterial disease which attacks the joints of the stem just above 
the ground, rotting the tissues and causing the plant to fall 
over. Besides this, there is the more common disease which at- 
tacks the roots of the plant. This latter disease is known to 
occur in various parts of the country and in many places is 
largely responsible for the low yields of corn. This rot is pro- 
duced by one or more species of fungi, w^hich are carried over 
from season to season in the seed. Tests made at the Experiment 
Station show that eighty to ninety per cent of the corn grains 
are affected by this disease. While most of the grains have the 




Pig-. 7. Showing- the difference in vigor of corn kernels from dif- 
ferent ears. This variation in vigor is partially due to the presence or 
absence of the root rot disease. 



18 



appearance of being perfectly healthy, the fungi can readily be 
cultured from them. 

Wliile this root rot always causes a serious loss, the severity 
of the attack depends to a considerable extent upon seasonal and 
weather conditions. In a cold, wet spring, the disease is much 
worse than it is in warm weather. Frequently, a badly infected 
field will recover rapidly during a period of warm, dry, growing 
weather. 

In the northern states, good results have been obtained by 
germinating kernels from every ear of seed corn and discarding 
all ears that do not show a strong germination. In, Fig. 7 are 
shown plants from three different ears illustrating the great 
variation that occurs in the vigor of the young plants. In 
Louisiana, we have never duplicated the results obtained in the 
northern states. In general, this seems to be due to our entirely 
different weather conditions and also to the fact that a large 
percentage of the weak plants are eliminated in the field when 
the corn is thinned. 

With us a possible solution of the problem ma^^. be the breed- 
ing of strains more or less resistant to the trouble. AVe now have 
corn varieties that only show a small amount of root injury, but 
unfortunately these varieties are among oui* lowest yielding ones. 
It may be possible to cross these varieties on some of our better 
corns and produce strains which are in all ways more desirable. 
Some preliminary crosses have been made during the season of* 
1921. We are using as one of the parents in the cross the AVhite 
Calhoun variety. This is a variety which has been developed 
by the Station within the past few years. 

OTHER PROBLEMS 

While it is necessary to confine the prhicipal part of the in- 
vestigational work to a few of the more important problems, 
other problems are being considered as time is available. It is 
the aim of the department to watch all of the different troubles 
and to obtain as much information regarding them as is possible. 
As the data accumulate, there will undoubtedly be developed 
better methods of control. Better control of these troubles means 
better crops and coiisequently a better agriculture. 



LIBRARY OF CONGRESS 

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